1. Field of the Invention
The present invention relates generally to a control system for an automotive automatic power transmission. More particularly, the invention relates to an automatic power transmission control system with simplified hydraulic circuit construction.
2. Description of the Background Art
Japanese patent First (unexamined) Publication (Tokkai) Showa 57-101152 discloses a control system for an automatic power transmission. The control system includes a solenoid valve designed for selecting or shifting gear position among four forward gear positions. For this, the shown solenoid valve generates mutually different four control pressures for controlling positions of shift valves. In the shown construction, four shift valves, i.e. 1-2 shift valve, 2-3 shift valve, 3-4 shift valve and 4th gear position clutch control valve, are provided in the hydraulic circuit of the automatic transmission. Respective four shift valves are adapted to respond to respectively corresponding ones of the control pressure to switch valve position.
Generally, the modern automatic power transmission includes various hydraulically operated devices for achieving various control functions. For example, one of the typical construction of the automatic power transmission includes a lock-up clutch control valve, a cut-back valve, a timing valve or valves, an overrunning clutch control valve and so forth. While the facilitating of various hydraulic devices such as that set forth above achieves additional functions in the automatic power transmission, the hydraulic circuit for controlling these hydraulic devices becomes more and more complicated to cause difficulty in designing and prevents a hydraulic control system from being constructed in a compact manner. For example, in the construction shown in the aforementioned Tokkai Showa 57-101152, another solenoid valve is required for controlling a lock-up clutch which is associated with a torque converter for establishing and releasing lock-up condition.
To simplify the construction of the hydraulic circuit, Japanese Patent First (unexamined) publication (Tokkai) Showa 59-183150 discloses a shift control solenoid valve which is commonly used for controlling a lock-up clutch control valve. In the shown construction, the shift control solenoid valve controls shift valves between a first gear position, -second gear position and a third gear position. In addition to this, the solenoid valve performs control operations for a lock-up clutch control valve for establishing and releasing a lock-up condition, while the shift valve is set at the third gear position. For facilitating this, the shift valve is constructed to be placed at the third gear position while no shift control pressure is exerted. With the shown construction, lock-up can be established only at the third gear position and cannot achieve full range lock-up control.
Furthermore, in the control system of the aforementioned Tokkai Showa 59-183150, difficulty is encountered in appropriately adjusting shift timing. In the shown control system, four mutually different control pressures are to be generated by the solenoid valve. Therefore, considering shifting from third gear position to second gear position, there are required two different pressure adjusting operations because 1-2 gear shift valve position is alternated between the lock-up released third gear position and lock-up third gear position. When the shift position is in the lock-up third gear position, the 1-2 shift valve is set at the second gear position, in which control pressure is exerted in an application chamber of a band servo device. On the other hand, when the shift position is in the lock-up released third gear position, the 1-2 shift valve is set at the first gear position, in which control pressure is not exerted in the application chamber of the band servo device. Therefore, in the latter case, the shifting operation from third gear position to second gear position is performed by draining pressure in a release chamber of the band servo device and by supplying the pressure to the application chamber. On the other hand, in the former case, it is only required to drain the pressure in the release chamber. In this connection, shift timing in the former case and the latter case has to be differentiated. For appropriately adjusting the shift timing, it becomes necessary for detecting not only the shift position but also lock-up clutch position.
On the other hand, Japanese Patent First (unexamined) publication (Tokkai) Showa 59-180149 discloses an automatic transmission control system which includes a pressure regulator valve, a cut-back valve, shift valves and solenoid valves. One solenoid valve is associated with an 1-2 shift valve for shifting gear position between first gear position and second gear position. The 1-2 shift valve is hydraulically communicated with the cut-back valve so that the cut-back valve can be indirectly controlled by the control pressure from the solenoid valve. However, since such construction cooperatively operates the 1-2 shift valve and the cut-back valve, lag in lowering line pressure tends to occur to cause uncomfortable shift shock. Namely, when lag in pressure adjustment in the pressure regulator valve occurs, the 1-2 shift valve starts to switch the valve position between first and second gear positions before the line pressure is sufficiently lowered for suppressing shift shock.